1. Field of the Invention
The present invention relates to a method of making a plurality of optical record disc substrates, and an apparatus therefore, wherein optical record disc substrates such as optical record discs and magnet-optic discs are formed, in particular a method of making a plurality of optical record disc substrates and an apparatus therefore, wherein at least two disc-shaped cavities are arranged in a mold, and optical record disc substrates can be made in multiple-molding.
2. Discussion of the Background
It has been known that a conventional optical record disc injection-molding process injects a molten resin into a mold with a cavity, compresses the injected resin by using an external force to lessen the volume of the cavity, and gives a predetermined shape to the compressed resin. The conventional means for lessening the volume of the cavity includes the use of a mold closing force of a molding device or the use of a hydraulic cylinder provided in a mold.
In such a type of optical disc molding, a single-molding mold has mostly been utilized because equal application of a mold closing force, and the presence of a radial flow where the flow of a resin in the cavity spreads out from the center to radial directions of a disc substrate are required in accordance with characteristics needed for the disc substrate.
However, such a single-molding mold involves inefficient productivity, and a plurality of molding devices are installed to increase a production capacity for increased production. This arrangement has a disadvantage in that improvement in cost is difficult. A multiple-molding process and an apparatus therefore which have efficiency productivity are coming into practice.
The multiple-molding according to a conventional inject-molding process involves a problem in that variations in charge among plural cavities have a direct effect to the molded product to create variations among them. This is because the volume of resin charge, the actuation timing of core bodies and cut punches, and the distribution of a mold closing force with respect to the plural cavities are controlled together as the entire mold without being independently controlled at the respective cavities. In other words, such variations in charge have a direct effect to the molded product to create variations in thickness and internal strain of the molded discs, thereby creating problems in that the discs warp and optical characteristics are poor. When a single mold is used to carry out a multiple-molding process wherein two record discs or more are molded simultaneously, a single stream of molten resin which has been injected from a molding device cannot be equally distributed to the two cavities or more. All gates at a molten resin inlet through which the distributed molten resin is injected into the cavities are difficult to be formed in the same sizes in terms of the structure of the mold. For these reasons, all cavities cannot have the molten resin injected therein under the same conditions, creating a problem in that all disc substrates cannot meet required characteristics.